FULL PRODUCT VERSION :
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Darwin leitang-MacBook-Pro.local 14.5.0 Darwin Kernel Version 14.5.0: Sun Sep 25 22:07:15 PDT 2016; root:xnu-2782.50.9~1/RELEASE_X86_64 x86_64

A DESCRIPTION OF THE PROBLEM :
public static void main(String[] args)
  {
    try {
      	String xx = "http://coder_1.tanglei.name";
        java.net.URI u = new java.net.URI(xx);
    	System.out.println(u);
    	System.out.println(u.getHost()); // null
      	java.net.URL url = new java.net.URL(xx);
        System.out.println(url);
    	System.out.println(url.getHost()); // coder_1.tanglei.name
        java.net.URI u2 = new java.net.URI(url.getProtocol(), url.getHost(), url.getPath(), "");
      	System.out.println(u2);
    } catch (Exception e){
      	System.out.println("exception got: " + e.getMessage());
    }
  }


"_" is not allowed in java.net.URI but is allowed in java.net.URL. 

output:

http://coder_1.tanglei.name
null
http://coder_1.tanglei.name
coder_1.tanglei.name
exception got: Illegal character in hostname at index 12: http://coder_1.tanglei.name#


REGRESSION.  Last worked in version 8u101

ADDITIONAL REGRESSION INFORMATION: 
java version "1.8.0_101"
Java(TM) SE Runtime Environment (build 1.8.0_101-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.101-b13, mixed mode)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
public static void main(String[] args)
  {
    try {
      	String xx = "http://coder_1.tanglei.name";
        java.net.URI u = new java.net.URI(xx);
    	System.out.println(u);
    	System.out.println(u.getHost()); // null
      	java.net.URL url = new java.net.URL(xx);
        System.out.println(url);
    	System.out.println(url.getHost()); // coder_1.tanglei.name
        java.net.URI u2 = new java.net.URI(url.getProtocol(), url.getHost(), url.getPath(), "");
      	System.out.println(u2);
    } catch (Exception e){
      	System.out.println("exception got: " + e.getMessage());
    }
  }


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The same as URL
ACTUAL -
http://coder_1.tanglei.name
null
http://coder_1.tanglei.name
coder_1.tanglei.name
exception got: Illegal character in hostname at index 12: http://coder_1.tanglei.name#

REPRODUCIBILITY :
This bug can be reproduced always.